1. Field of the Invention
The present invention relates to liquid crystal display and more particularly to a backlit field sequential color display system.
2. Description of the Prior Art
The most widely used method of generating electrically-controlled color images uses a cathode ray tube (CRT) having a matrix of phosphor stripes or dots representing three primary colors and three electron guns which excite respectively differently ones of the color phosphors. Because the different colors are scanned simultaneously, and color separation depends upon position, this type of system is known as an "area multiplexed" video display system. Display systems of this type are advantageous because they are bright, relatively inexpensive and based on well-understood technology. However, CRTs are not suitable for use in every situation due to their relatively high power consumption, bulk, weight, and fragility.
Monochrome CRTs, that is CRTs having a uniform white phosphor coating, were used in an early type of color display system known as a "field sequential" display. This system was introduced by CBS Laboratories in the late 1940s for use in commercial television broadcast equipment. The CBS system sent distinct sequential red, green and blue image fields, in sequence, to the TV receiver. The receiver had a rotating color filter wheel in front of a single, white monochrome CRT screen. Synchronization of the three fields with the rotation of the filter wheel resulted in a color video display.
Recently, monochrome CRTs have been combined with nonmoving, switchable color filters covering the CRT screen to yield colored images by this same principle. Typically, three distinct white images are displayed on the CRT screen in sequence. The color transmitted by the switchable filter is synchronized with the sequence of images. This creates a repeating sequence of distinct colored images (for example, red, green, blue, red, green, blue, . . . ). If the sequence rate (or "refresh rate") is high enough, the separate images combine in the eye of the viewer to make one colored image. The desirable frame rate which displays a flicker-free color image is approximately three times the frame rate for an area multiplexed system (e.g. 180 Hz, versus 60 Hz).
The switchable filters in these systems include: color selective linear polarizing filters in conjunction with twisted nematic liquid crystals (U.S. Pat. No. 4,758,818 to Vatne, Jul. 19, 1988; IBM Technical Disclosure Bulletin, Vol. 22, No. 5, October 1979, pp. 1769-1772, by Brinson and Edgar); liquid crystals which include pleochroic dyes (U.S. Pat. No. 3,703,329 of Castellano); and liquid crystals with birefringent materials (U.S. Pat. No. 4,097,128 of Matsumoto).
While avoiding the complexity and alignment problems of color matrix CRTs, these systems suffer from the drawbacks inherent in a monochrome CRT such as bulk, weight and power requirements. In addition, the color filters used in these systems, which may include as many as five or six panels, absorb a large proportion of the light emitted by the CRT, thus requiring a very bright CRT screen if brightness equivalent to an area-multiplexed display is to be maintained.
Because of these drawbacks, these CRT-based color display systems may be replaced by a new type of video display for certain applications. This new type of display is based upon a panel comprised of an array of light valves, each light valve corresponding to a single picture element, or pixel. When the array is illuminated from behind, typically by florescent or incandescent lamps, images can be formed by selectively opening and closing certain of the valves in the array. In displays of this type, the light source does not form the image directly; it should in fact be diffuse, so as to be evenly transmitted through the light valve array.
The most commonly used light valve now available is the liquid crystal (LC). Liquid crystal devices (LCDs) may be used to produce light-weight, compact, relatively high contrast display panels that use much less power than otherwise equivalent CRT display devices. These display panels are particularly useful as display devices in a portable computer or in an airplane cockpit where weight and power consumption are critical.
In a typical monochrome application, a matrix of LCDs, each occupying the area of one pixel, are constructed with integral thin film transistors (TFTs). The TFTs are individually addressed by circuitry external to the display device. When a TFT is addressed, it opens its associated LCD light valve allowing the back-light to pass through that pixel position in the array. When a pattern of LCD light valves is energized, an image is displayed on the LCD matrix. A display of the type described above is generally referred to as an LCD display. When the TFTs in an LCD display are controlled to only partially open their respective light valves, the image may include greyscale information. A system of this type is described in U.S. Pat. No. 4,742,346 to Gillette et al.
Monochrome LCD displays are widely used for computer displays and for video displays in small television sets. Recently, there has been a growing interest in color LCD displays. To be practical, a color LCD display should have brightness and longevity which approximates that of a color CRT and a refresh rate high enough to show smoothly moving images without flicker.
Currently, there are several types of color LCD display. One type of display uses LCDs in a consumer-market video projector. In this projector, the LCDs are not viewed directly, but act as does film in a movie projector. Steady-output colored light sources illuminate respectively different LCDs and the separate colored images produced by the LCDs are combined and projected onto a screen. In an exemplary LCD projector, the XV-100P projector manufactured by Sharp Corporation, there are three light sources (red, blue, and green) and three LCDs. Dichroic mirrors are used to separate white light into the three light sources and also to combine the separate images produced by the three LCDs into a single image. This image may be projected onto a screen using conventional optical lenses.
A hybrid display system using both CRTs and an LCD matrix uses three monochrome color CRTs (red, green, and blue) as light sources. Light from each CRT screen is picked up by a separate bundle of optical fibers fastened to the face of the CRT. The fibers of the bundles attached to the respective red, green, and blue CRTs are then alternatingly intermixed at their termination, which is fastened to the thin edge of a plastic light-diffusing plate. This plate is designed to provide uniform diffused illumination from its side when the edge is illuminated by the fiber bundle. The three CRTs are activated sequentially at an individual rate of 30 Hz; the combined rate is 90 Hz. An LCD panel covering the illuminating side of the plastic diffusing plate forms a field sequential image by spatially modulating the red, green, and blue images transmitted by the LCD. The generation of red, green and blue images on the LCD panel is synchronized with the light flashes from the respective CRTs.
This system is described in an article by H. Hasabe and S. Kobayashi entitled "A Full-Color Field-Sequential LCD Using Modulated Backlight," in the SID 85 Digest, pp.81-83. It uses a thin display panel, but requires three optical fiber bundles and uses three CRTs. Light is lost in passing from the CRT screen into the fiber array and through the diffusing panel, as well as in the LCD matrix. This display system is described as having a brightness of only 30 foot-lamberts (fL) as compared to 100 fL for a CRT display. Moreover, since this display system uses CRTs to produce the backlight, it suffers from the bulk, weight, and power consumption problems of conventional CRT displays.
Other prior-art designs for color LCD panels employ a white light source and an LCD panel tessellated with sub-pixel size color filter elements. Each individual filter element is aligned to cover, for example, one-third of one image pixel. Each pixel includes three separate TFTs and three respectively different primary color filter elements. Display systems of this type are referred to as area multiplexed color systems. The three primary color images are displayed simultaneously by energizing the respective sub-pixel image elements. An area multiplexed display of this type is described in an article by G. Stix, entitled "Manufacturing hurdles challenge large-LCD developers" in the IEEE Spectrum, September 1989, pp 36-40.
Area multiplexed LCD panels are less efficient than monochrome panels because of the color filter elements. Typical filter elements used in these displays pass only about twenty percent (20%) of the incident light.
Another consideration is that the distance between the pixels should be small enough so that the eye of the viewer does not perceive the image as excessively grainy. The individual sub-pixel elements of any particular color are not adjacent in area multiplexed systems; each pixel contains all three primary color sub-pixels and desirably has a cell dimension that does not exceed the grainy limit. This means that the sub-pixels must be smaller, by a factor of at least the square root of three, than the pixels. Thus, a portion of an image which is composed of a single color is dimmer by at least a factor of the square root of three than a white portion of the image.
In addition, each of these sub-pixels has a separate energizing TFT with associated addressing lines. These elements tend to decrease the fill factor of the display (the fill factor is defined as the proportion of panel area which transmits light). This reduced fill factor further reduces the brightness of an area-multiplexed LCD display as compared to a monochrome LCD array.
Since the filter mosaic is integral with the LCD panel, the individual filter elements are desirably aligned very closely with their associated sub-pixels. The presence of these filter elements restricts the high-temperature processing steps which may be performed on the LCD array. Furthermore, the chemical dyes which are used to form the filter elements are subject to degradation over time.
A further disadvantage of area multiplexed screens is the appearance of "artifacts" in the image due to the smaller size of the sub-pixels as compared to monochrome pixels. One type of artifact is caused by "charge-sharing" between adjacent pixels. The liquid crystal material in an LCD is energized by placing an electric field across the material in the pixel area. Since the sub-pixel elements are relatively small, charge from one sub-pixel may affect the liquid crystal material beneath an adjacent sub-pixel. This may affect the purity of the displayed color.